This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-108620, filed Apr. 6, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a spin valve transistor for use in reading magnetic heads and the like for magnetic recording.
2. Description of the Related Art
In recent years, spin valve transistors (spin tunnel transistors) that have a large magneto-current ratio (MR ratio) have come to attract attention as magnetic sensors for reading data from magnetic discs.
FIG. 4 is a cross-sectional view, or a schematic representation of the structure of a conventional spin valve transistor.
The spin valve transistor depicted in FIG. 4 comprises a collector region 1 (semiconductor layer 11), a base region 2 (ferromagnetic layer 12, nonmagnetic layer 13 and ferromagnetic layer 15), a barrier layer 3 (insulating layer 14 for a tunnel (or a semiconductor layer for a Schottky barrier)), and an emitter region 4 (electrode layer 17). As FIG. 4 shows, a metal layer 19 is provided on the ferromagnetic layer 12. The metal layer 19 may be dispensed with, nonetheless.
Of the two ferromagnetic layers 12 and 15 provided in the base region 2, the lower ferromagnetic layer 15 is fixed in magnetization direction. On the other hand, the upper ferromagnetic layer 12 is not fixed in magnetization direction; its magnetization direction changes in accordance with the direction of an external magnetic field emanating from a magnetic disc or the like. The electron current flowing from the emitter region 4 to the collector region 1 through the base region 2 is larger when the upper ferromagnetic layer 12 has the same magnetization direction as the lower ferromagnetic layer 15 than when the layer 12 has the opposite magnetization direction. Hence, the magnetization direction of the magnetic disc or the like can be determined by detecting the magnitude of this current.
The spin valve transistor can acquire an extremely large MR ratio of several hundred percent. However, it is disadvantageous in that the collector current is very small, about 10xe2x88x924 of the emitter current. The small ratio of the collector current to the emitter current is not desirable in view of power consumption, operating speed and noise.
The ratio of the collector current to the emitter current (i.e., current gain) is small, because the hot electrons injected into the base region undergo diffuse scattering in the ferromagnetic layer or at the interface between the ferromagnetic layer and the nonmagnetic layer. Once subjected to diffuse scattering, hot electrons cannot move into the collector region and move from the base region to the outside of the element. Therefore, the diffuse scattering in the ferromagnetic layer or at the interface between the ferromagnetic layer and the nonmagnetic layer should be suppressed in order to increase the collector current.
The diffuse scattering in the ferromagnetic layer can be suppressed by reducing the thickness of the ferromagnetic layer. In the spin valve transistor, however, many interfaces exist since the base layer has two ferromagnetic layers 12 and 15. It is therefore difficult to suppress the diffuse scattering at the interface. The probability of the diffuse scattering exponentially increases with the number of interfaces. Thus, the diffuse scattering at the interface makes a great problem with the spin valve transistor.
Most spin valve transistors must have an antiferromagnetic layer in order to fix the magnetization direction of the ferromagnetic layer. In the conventional spin valve transistor described above, the diffuse scattering in the base region will increase even more if an antiferromagnetic layer is provided on the ferromagnetic layer 15. The collector current will inevitably decrease very much. Conversely, the antiferromagnetic layer may be provided beneath the ferromagnetic layer. In this case, the antiferromagnetic layer must not cover the region through which electrons move into the collector region. Consequently, the magnetization direction of the ferromagnetic layer cannot be firmly fixed, and the transistor cannot exhibit a stable output characteristic.
As specified above, the conventional spin valve transistor cannot have a large current gain or a stable operating characteristic, because any ferromagnetic layer is provided in the base region. It is therefore desired that a spin valve transistor that has a large current gain and a stable operating characteristic.
According to an aspect of the invention, there is provided a spin valve transistor that comprises a collector region made of semiconductor; a base region provided on the collector region and including a first ferromagnetic layer whose magnetization direction changes in-accordance with a direction of an external magnetic field; a barrier layer provided on the base layer and made of insulator or semiconductor; and an emitter region provided on the barrier layer and including a second ferromagnetic layer whose magnetization direction is fixed.